The serotonin system (5-HT) may stimulate ventilation in the brain. Evidence exists that ventilatory stimulation is helpful for patients with obstructive sleep apnea (OSA) and that the specific 5-HT agonist, fluoxetine, is helpful in some, but not all patients with OSA. We hypothesize that the serotonin system may be down-regulated or hypoactive in those patients who respond to fluoxetine, and normal in non-responders.